Over the last four decades, the density of integrated circuits has increased by a relation known as Moore's law. Stated simply, Moore's law says that the number of transistors on integrated circuits (ICs) doubles approximately every 18 months. Thus, as long as the semiconductor industry can continue to uphold this simple “law,” ICs double in speed and power approximately every 18 months. In large part, this remarkable increase in the speed and power of ICs has ushered in the dawn of today's information age.
Unlike laws of nature, which hold true regardless of mankind's activities, Moore's law only holds true only so long as innovators can continue to overcome the technological challenges associated with it. Improvements in optical overlay metrology, which allows extremely accurate overlay measurements to be performed between patterned layers on an IC, are being continuingly developed to allow Moore's law to continue to hold true. Optical overlay metrology can measure how accurately an overlying patterned layer, such as a photoresist layer, aligns with respect to an underlying patterned layer, such as a dielectric or metal layer, for example. If a workpiece contains layers that are misaligned, then the misaligned layers can be re-worked to correct the misalignment. For example, a misaligned photoresist layer can be stripped, and a new layer of photoresist can be applied and re-exposed at a corrected alignment, thereby enabling precise alignment of IC features.